In recent years, in order to achieve a higher breakdown voltage and lower loss of a semiconductor device, use thereof in an environment at high temperature and the like, silicon carbide has increasingly been adopted as a material for forming a semiconductor device. Silicon carbide is a wide band-gap semiconductor greater in band gap than silicon conventionally widely used as a material for forming a semiconductor device. Therefore, by adopting silicon carbide as a material for forming a semiconductor device, a higher breakdown voltage, a lower ON resistance of a semiconductor device and the like can be achieved. In addition, a semiconductor device adopting silicon carbide as a material is also more advantageous than a semiconductor device adopting silicon as a material in that deterioration in its characteristics at the time when it is used in an environment at high temperature is less.
Among such semiconductor devices including silicon carbide as a material, with regard to such a semiconductor device as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or an IGBT (Insulated Gate Bipolar Transistor) in which whether or not an inversion layer is formed in a channel region directly under a gate insulating film with a prescribed voltage serving as a threshold is controlled so that a current is allowed to flow or cut off, various studies on adjustment of a threshold voltage or improvement in channel mobility have been conducted (see, for example, Sei-Hyung Ryu et al., “Critical Issues for MOS Based Power Devices in 4H-SiC,” Materials Science Forum, 2009, Vols. 615-617, p 743-748 (NPL 1)).